1. Field of the Invention
The present invention relates to a method for production of a nonreducible dielectric ceramic composition and, more particularly, it relates to a method for production of a nonreducible dielectric ceramic composition suitable as a dielectric material for monolithic ceramic capacitors of the kind wherein internal electrodes are of a base metal such as, for example, nickel.
2. Description of the Prior Art
In general, dielectric ceramic materials of the prior are reduced to a semiconductor when fired in a neutral or reducing atmosphere with a low-oxygen partial pressure. Thus, it is essential for such a dielectric material to fire in an oxidizing atmosphere. If such a dielectric ceramic material is applied to monolithic ceramic capacitors, a material for internal electrodes to be used is required to have a high melting point and high resistance to oxidation at a high sintering temperature of not less than 1100 .degree. C. To this end, noble metals such as palladium, platinum have widely been used as a material for internal electrodes of the monolithic ceramic capacitors, but use of such a noble metal has become a bar to reduction of the cost of monolithic ceramic capacitors.
To solve such a problem, it has been proposed to use inexpensive base metals such as nickel as a material for internal electrodes. Since such a material is easily oxidized in the conventional sintering atmosphere, it is required to fire the ceramic material in neutral or reducing atmospheres with a low partial pressure of oxygen to prevent internal electrodes from oxidation during sintering. It is therefore required to use a nonreducible dielectric ceramic material which is not semiconductorized even if fired in a neutral or reducing atmosphere with a low partial pressure of oxygen, and which possesses high insulating resistance and excellent dielectric properties enough to use it as a dielectric material for capacitors.
Japanese patent publication No. Hei 2-63664 discloses a nonreducible dielectric ceramic composition consisting essentially of barium titanate and containing Ce incorporated therein. This material is never reduced even in a reducing atmosphere, and has small grain size and high dielectric constant.
Such a dielectric ceramic material has generally been manufactured by the conventional powder method employing carbonates and/or oxides of the respective components as raw materials. In this method, the raw materials are mixed in given molar ratios, milled the wet process and then calcined.
However, it is difficult with the conventional method to produce a ceramic material with a uniform composition as grain sizes of raw materials can not be reduced to less than 1 .mu.m. Thus, if the powder method of the prior art is applied to production of a solid solution of barium titanate containing a rare earth element, atoms of the rare earth element do not diffuse uniformly into the solid solution by calcination, resulting in the local increase in the concentration of the rare earth element.
As is known, barium titanate is changed from an insulator to a semiconductor by the presence of an excess amount of the rare earth element. Thus, the local increase of the rare earth element concentration causes production of a partially semiconductorized solid solution of barium titanate, resulting in decrease in the reliability of the monolithic ceramic capacitors.